1. Field of the Invention
The present invention relates to an error correction method for optical discs, and more particularly, to an error correction method appropriate for high density discs.
2. Description of the Related Art
There are currently a variety of optical discs available, including a compact disc (CD), a digital versatile disc (DVD), and a high density DVD (HD-DVD), which requires higher density recording and reproducing than a DVD, and is currently under development. While a conventional DVD has a storage capacity of 4.7 GB, the HD-DVD has a storage capacity of 15 GB or more. The higher storage capacity of the HD DVD is implemented by reducing the diameter of a beam spot for data recording/reproducing and increasing the line density.
The amount of data affected by a defect in an HD-DVD is far greater than the amount of data affected by the same length defect in a conventional DVD. Therefore, an HD-DVD requires stronger error correction than a conventional DVD.
FIG. 1 shows the structure of an error correction code (ECC) block in a conventional DVD. The error correction code block shown in FIG. 1 has a 10 byte parity for error correction of 172 bytes of data in the row direction, as an inner parity (PI), and a 16-byte parity for error correction of 192 bytes of data in the column direction, as an outer parity (PO). Here, the capability of error correction by the PI is a maximum of 5 bytes, and that of the PO is a maximum of 16 bytes for erasure correction.
Assuming that an HD-DVD uses the same error correction method as a conventional DVD, the effect of a defect will now be explained in detail.
FIG. 2 illustrates the relationship of a beam spot and an object lens in an optical disc.
Table 1 illustrates the relationships among t, the thickness of a disc, NA, the numerical aperture of an object lens, 2R, the diameter of a beam spot, and k, the length of a defect.
TABLE 1t (mm)NAR (mm)2R (mm)Remarkk, length of defect0.60.60.2480.496DVDk + 2R0.650.2730.5460.30.650.1360.2720.850.1930.2860.20.850.1290.2580.10.70.0490.098DVD/3.880.850.0640.128
1) The effect of a large defect
Here, a large defect means a burst error which cannot be corrected by a PI, and is generated by a scratch, a finger print, a black dot, etc.
A defect which spans 5 bytes or more is a burst error which cannot be corrected by a PI. At this time, the length of a defect is k=5 bytes×16 channel bits×0.133 μm (the length of 1 channel bit)=10.64 μm.
When a 20 GB HD-DVD is compared to a 4.7 GB DVD, the line density increase is (20/4.7)1/2. Accordingly, the same length defect damages 2.1 times more data in an HD-DVD than in a DVD.
Though an HD-DVD seems to be more advantageous than a DVD due to the HD-DVD's smaller spot size, the stabilization time required for restoring a reproduction signal (RF) in an HD-DVD is longer. Therefore, the effect of a spot size is thought to be similar in an HD-DVD and a DVD.
2) The effect of a small defect
Here, a small defect means a burst error which can be corrected by a PI, and is generated by dust and the like. The length of the defect is equal to or less than 10.64 μm.
According to table 1, when NA=0.85 and t=0.1 mm, the diameter of a beam spot incident upon the surface of an HD-DVD is 0.128 μm, which is 1/3.88 times that of a DVD with a diameter of the beam spot being 0.496 μm. Therefore, the HD-DVD's probability of error occurrence by a small defect becomes 3.88 times greater than that of a DVD.
In addition, since the line density of an HD-DVD is 2.1 times greater than that of a conventional DVD, the probability of error in an HD-DVD is 8.148 times (3.88×2.1=8.148) greater than that of a DVD for the same size defect. This means that when an HD-DVD uses the same modulation method as a DVD, error correction by a PI must be available for about 40.74 bytes (5 bytes×8.148). Therefore, an HD-DVD requires a great number of PIs.
In the previous DVD error correction method shown in FIG. 1, in order to raise the burst error correction capability, the number of data columns must be increased in the PI direction, while the number of data rows must be decreased in the PO direction.
However, when n, the number of data columns in the PI direction, exceeds 256, a Galois Field operation GF(28) cannot be performed.
Thus, the previous error correction method in a DVD as shown in FIG. 1 cannot be easily applied to HD-DVD.